For example, in a full color copying machine such as shown in FIG. 17, a full color copying operation is performed as follows. Scanning is executed by an exposure lamp 102 of an optical system 101 three times with respect to a color original document 103, and light reflected from the color original document 103 in each of the scans is independently transmitted through a predetermined one of three filters having respective three colors of red, green, and blue in a filter device 104, thereby irradiating a photoconductor 105. Thus, the photoconductor 105 is exposed, and three electrostatic latent images are formed on the photoconductor 105. Subsequently, each of the electrostatic latent images obtained through the above three exposures is independently developed by respective color toner which is one of three colored developers of yellow, magenta and cyan, stored in developing devices 106 to 108, and the resulting toner images are successively transferred onto an intermediate transfer belt 109, being overlapped thereon one upon another to form a toner image. Next, the toner image on the intermediate transfer belt 109 is transferred onto a copy paper sheet (not shown), and a color picture image is obtained on the copy paper sheet by fixing the toner image on the copy paper sheet through heat treatment at a fixing device 110.
The above transfer operation of the toner image from the intermediate transfer belt 109 to the copy paper sheet is performed by a back-up roller 111 made of rubber such as insulating silicon rubber, a rear-surface electrode roller 112 of driven type, disposed at a vicinity of the back-up roller 111, and a metal transfer roller 113 disposed confronting the back-up roller 111, with the intermediate transfer belt 109 interposed in between. More concretely, the intermediate transfer belt 109 is supported from its back side by the back-up roller 111 and the rear-surface electrode roller 112, and while supplying a copy paper sheet so as to superpose it on the toner image on the intermediate transfer belt 109, the transfer roller 113 and the back-up roller 111 depress the copy paper sheet onto the intermediate transfer belt 109. Then, under the above condition, as shown in FIG. 18, a transfer voltage is applied between the transfer roller 113 and the rear-surface electrode roller 112 from a power supply 115, thereby transferring the toner image formed on the intermediate transfer belt 109 onto the copy paper sheet 114.
However, the conventional arrangement results in the following problems when transferring the toner image formed on the intermediate transfer belt 109 onto the copy paper sheet 114.
More specifically, in the transfer operation, when the transfer voltage is applied between the transfer roller 113 and the rear-surface electrode roller 112, a current I.sub.1 of substantial 50 to 100 .mu.A flows between the transfer roller 113 and the rear-surface electrode roller 112 in a portion having the intermediate transfer belt 109 and the copy paper sheet 114 interposed therebetween, for example, in the case of applying a transfer voltage of 2 kV, as shown in FIG. 19 (a), since there exist a resistance R.sub.TX of the intermediate transfer belt 109 and a resistance R.sub.p of the copy paper sheet 114 between the rollers 113 and 112. On the other hand, in a portion between the transfer roller 113 and the rear-surface electrode roller 112, having only the intermediate transfer belt 109 interposed therebetween, that is, having no copy paper sheet 114 interposed therebetween, an excess current I.sub.2 of substantial 2 mA flows due to a drop of the total resistance of the circuit caused by the absence of the resistance R.sub.p of the copy paper sheet 114, as shown in FIG. 19 (b). For that reason, in the case where there is some toner remaining on the intermediate transfer belt 109 between the transfer roller 113 and the rear-surface electrode roller 112 in a portion having no copy paper sheet 114 interposed therebetween, the toner adheres on the intermediate transfer belt 109 due to heat generated by an excess current flowing through the intermediate transfer belt 109, causing a so-called filming phenomenon, and as a result copy quality is adversely affected.
On the other hand, in the full color copying machine, copy paper sheets 114 of various kinds and sizes, for example, such as sheets for OHP (Over Head Projector), (hereinafter called OHP sheets), A-3 size, or A-4 size copy paper sheets, are used. Therefore, in transfer operation from the intermediate transfer belt 109 to the copy paper sheet 114, depending on the kind and size of the copy paper sheet 114, one of modes, such as OHP mode, A-3 mode or A-4 mode is selected so as to set a travel speed of the intermediate transfer belt 109, that is, a process speed, to 60 mm/s, 115 mm/s, or 184 mm/s respectively.
However, since the process speed in transfer operation is different depending on each mode to be set according to the kind and size of the copy paper sheet 114, an optimum value of the transfer voltage is different in each of the modes. Therefore, a different transfer voltage should be set for each of the modes in order to obtain a desirable transfer operation, and for that reason it is necessary to install expensive transformers for the respective modes, resulting in a high manufacturing cost.